Conventionally, tool steels have been widely used for forming a mold (such as trimming, die, or drawing) for cold forging, precision forging, progressive press, plastic molding, warm forging, powder molding and magnet molding, and mold parts attached to the mold.
Tool steels are materials which are required to have high hardness and hence, the structure of the tool steels is transformed into martensite by applying quenching and tempering to them so as to impart desired hardness, and such tool steels are used as materials of the above-mentioned mold or the like.
Tool steels expand a volume thereof due to quenching and tempering. Although there arises no problem when the expansion is an isotropic expansion, conventional cold work tool steels generate anisotropic and non-uniform expansion thus giving rise to a serious problem in the manufacture of a mold or the like.
This anisotropic and non-uniform expansion of tool steels is liable to conspicuously appear particularly with respect to tool steels containing a large quantity of carbide. However, the reason of such a phenomenon has not been clarified yet.
The anisotropic and non-uniform expansion of tool steels gives rise to a following problem in the manufacture of a mold, for example.
In the manufacture of a mold, a tool steel is roughly formed into a rough mold having a shape and a size which are preliminarily estimated by adding a size change to be caused by heat treatment to a desired mold size and, thereafter, quenching and tempering are applied to the rough mold and, finally, finish working is applied thereto to form a mold having a desired shape.
In the case that the mold material (tool steel) generates an isotropic expansion thereof due to quenching and tempering, the mold may be roughly formed into a size and a shape allowing the expansion of equal quantity in all directions.
However, when the mold material extends (expands) largely in one direction while extends little or contracts in another direction due to quenching and tempering, it is necessary to determine the size of the mold material before quenching and tempering with taking a size change in such another direction into consideration.
However, the direction that the mold material extends due to quenching and tempering also differs depending on the direction along which a material to be the mold is taken out from a raw material. Therefore, there is no reproducibility of size after quenching and tempering and the size of the mold cannot be controlled with desired accuracy. This drawback largely hampers the manufacture of the mold.
Accordingly, for example, compared with mold size accuracy of ±0.03% (size accuracy of ±30 μm when a length of the mold is 100 mm) which is required to be satisfied by general users, a size of the mold before heat treatment is conventionally made uniformly large (approximately +0.06%) so that even when the size cannot be controlled due to quenching and tempering (+0.06±0.03%=0.03% to 0.09%), a sufficient machining margin (+0.03% or more, and 1 to 30 μm being removed when the machining margin by cutting is less than 0.03%, and this cutting being difficult from a viewpoint of rigidity of a machine or the like) is ensured.
However, in this case, a machining margin of finish working becomes 0.09% at maximum and, at the same time, tool steel is basically a material which has high hardness and hence, working after heat treatment requires a considerably long time (assuming that cutting is performed for every 0.03%, it is necessary to perform cutting three times).
Alternatively, there also arises a serious drawback that a load which a cutting tool receives is excessively increased (when the working margin of 0.09% being worked one time), leading to breaking of the cutting tool.
Accordingly, there has been a strong demand for the reduction of machining margin. However, factors which controls the non-uniformity of expansion due to heat treatment has not been revealed and hence, no countermeasure has been found up to now.
JP-A-2005-113161 discloses a technique which aims at solving a problem on anisotropy of thermal expansion ratio in hot work tool steels. In this case, the thermal expansion ratio is a ratio at which the material to which heat treatment of quenching and tempering is applied (with no phase transformation) expands corresponding to a temperature.
The present invention relates to a heat treatment in quenching and tempering, that is, the isotropy of a size change of a tool steel when the phase transformation is generated. Therefore, the present invention fundamentally differs from the technique disclosed in JP-A-2005-113161 with respect to a point that the phase transformation is present or not. Accordingly, the isotropy of the size change of the tool steel of the present invention when the phase transformation is generated should not be estimated by this document.
Further, JP-A-2003-226939 discloses a technique which improves machinability by controlling particle sizes and quantities of carbide and non-metallic inclusions in hot work tool steel.
However, this document fails to disclose the problems to be solved by the present invention, and the present invention also differs from the technique disclosed in this document with respect to a technique for overcoming the problems.